1. Field of the Invention
The invention relates to the production of high surface area alpha-alumina suitable as a catalyst and catalyst support and, in particular, to the production of high surface area alpha-alumina having high temperature stability.
2. Description of the Prior Art
Aluminum oxide, or alumina, in several forms (anhydride or hydrated, crystalline or amorphous) has been known for many years. Various forms of alumina occur in nature and many have been produced synthetically. Certain forms of alumina have proven outstanding for use as catalysts and as catalyst supports and consequently have found extensive use in the petroleum refining industry. For example, the gamma form of alumina (.gamma.-Al.sub.2 O.sub.3), composited with a noble metal or mixture of such metals with each other or with other metals, has been widely used in reforming processes because, inter alia, of its high surface area, heat stability and surface acidity and because it can be prepared in very pure form.
Alpha-alumina (.gamma.-Al.sub.2 O.sub.3) is a chemically and thermally stable form of alumina which has been known for many years. It is well-known that all other forms of alumina can be converted to alpha-alumina; see, e.g. "Oxides and Hydroxides of Alumina", K. Wefers et al, Tech Paper No. 19, Alcoa Research Laboratories (1972), pp. 36-45. Alpha-alumina is widely available and can be prepared in very pure form. Because of its high thermal stability, alpha-alumina is particularly widely used in the manufacture of ceramics. However, despite the wide availability and its unusually high thermal stability, alpha-alumina has found little use in petroleum refining operations because of its relatively low surface area (typically less than about 2 m.sup.2 /g).
In recent years, efforts have been devoted to developing processes for producing alpha-alumina having a high surface area in order to take advantage of its high thermal stability properties for use in petroleum refining operations. Exemplary of such developments are U.S. Pat. Nos. 4,012,337, 3,908,002 and 3,726,811. The latter patent describes the preparation of an alpha-alumina with a surface area of at least 40 m.sup.2 /g. This is achieved by impregnating an alumina xerogel with a minor amount of a carbonizable material, e.g., a starch or sugar such as sucrose. Impregnation is followed by carbonization, conversion to the alpha-phase and finally removal of carbon. The alpha-alumina thus produced may be used as a catalyst or catalyst support in processes that are carried out at about 800.degree. C. or lower.
U.S. Pat. No. 3,726,811 represents an advance in the art of producing high surface area alpha-alumina, but nonetheless suffers from two disadvantages. First, the conversion to alpha-alumina must be carried out at a very high temperature--greater than 1300.degree. C. Second, the alpha-alumina thus produced is useful in catalytic reactions occurring only at temperatures of about 800.degree. C. and lower. While this limitation is of little consequence in most catalytic processes, there are some catalytic processes which require higher temperatures, typically on the order of 1000.degree. C. or so. Such reactions include resid cat cracking, which requires regeneration temperatures of about 800.degree. to 1000.degree. C. and steam reforming, a severe process which requires temperatures of greater than 800.degree. C.